Porous silica films are potentially useful as low dielectric constant intermetal materials in semiconductor devices, as low dielectric constant coatings on fibers and other structures, and in catalytic supports. Most of the U.S. semiconductor industry is presently (1998) in the process of implementing interlevel dielectric films that are silica films, or derivatives of silica and silicates, or polymeric films, with less than 25% or no porosity with dielectric constant (k') in the range of 3.0 to 4.0. Further reductions in dielectric constant are desired to improve the operating speed of semiconductor devices, reduce power consumption in semiconductor devices and reduce overall cost of semiconductor devices by decreasing the number of metallization levels that are required.
Since air has a dielectric constant of 1.0, the introduction of porosity is an effective way of lowering the dielectric constant of a film. In addition, because silica dielectrics have been a standard in microelectronic devices, silica films with porosity are very attractive to the semiconductor industry for advanced devices that require low dielectric constant materials. The feature size or design rule in the semiconductor interconnect will be sub-150 nm in ultralarge scale integration; and pore sizes to achieve lower dielectric constant (k&lt;3) must be significantly smaller than the intermetal spacing.
Dielectric constant of porous films is dependent on the material and pore structure. For porous silica films for use in microelectronic devices, material and pore structure must result in uniform dielectric constants across the wafers and in different directions on the wafer. In general, isotropic material and pore structures are expected to provide the desired uniformity in film dielectric constant compared to anisotropic material and pore structures.
Also, low dielectric constant mesoporous films used commercially need to be prepared in a manner compatible with a semiconductor device manufacturing process line, for example spin coating. For large-area circular wafers, other coating techniques such as dip coating are not as convenient because dip coating requires masking of the backside to prevent contamination.
Surface topography is also very critical to fabrication of a multilevel interconnect structure. In the "damascene" process for copper interconnects intended for ultralarge scale integration on semiconductor chips, each dielectric layer is etched, following which copper is deposited, and the structure planarized by chemical-mechanical polishing (CMP). The initial planarity and the absence of surface texture in the low k dielectric film is very critical in maintaining planarity at each level of the interconnect.
Another important concern with porous dielectric films is mechanical integrity. Because of their fragility, it appears unlikely that porous films will be directly polished using conventional chemical-mechanical-polishing (CMP) equipment, but a dense "cap" layer of silica or another material on the porous low K film will be planarized. However, even with a cap layer, the porous low K material must have adequate stiffness, compressive and shear strengths, to withstand the stresses associated with the CMP process.
Silica films with nanometer-scale (or mesoporous) porosity may be produced from solution precursors and classified into two types (1) "aerogel or xerogel" films (aerogelixerogel) in which a random or disordered porosity is introduced by controlled removal of an alcohol-type solvent, and (2) "mesoporous" surfactant-templated silica films in which the pores are formed with ordered porosity by removal of a surfactant. Heretofore, the most successful demonstration of low dielectric constant silica films with dielectric constant of 3.0 or less has been with aerogel/xerogel-type porous silica films. However, disadvantages of aerogel/xerogel films include (1) deposition of aerogel/xerogel films requires careful control of alcohol removal (e.g. maintaining a controlled atmosphere containing solvent or gelling agent during preparation) for formation of the pore structure (2) the smallest pore size typically possible in aerogel/xerogel films falls in the size range of 10-100 nm, and (3) limited mechanical strength compared to dense selica films. These disadvantages have hindered implementation of these aerogel/xerogel porous silica films in semiconductor devices.
In order to obtain a porous film with a low dielectric constant of any material made by any process, it is necessary to minimize the number of hydroxyl groups in the structure, especially at pore surfaces. The dielectric films must be made hydrophobic in order for the electrical properties to be stable in humid air. Hydroxylated surfaces in porous silica films result in a dielectric constant exceeding that of dense silica (i.e. approximately 4.0). Physisorption of water molecules by hydroxylated surfaces can further increase the dielectric constant and effective capacitance of a mesoporous silica film. Physisorption of water molecules can be avoided by handling films in non-humid atmospheres or vacuum, or by minimizing exposure of films to humid conditions. Hydroxyl groups and physisorbed water molecules may be removed from silica surfaces at very high temperatures. C. J. Brinker and G. W. Scherer, in Sol-Gel Science, Academic Press, New York, N.Y. (1990) (Brinker et al. 1990) discuss thermal dehydroxylation of silica by exposure to very high temperatures of over 800.degree. C. However, semiconductor devices with dielectric films and metal lines cannot usually be processed over about 500.degree. C. Thus, other methods of dehydroxylation are needed for porous silica films on semiconductors.
E. F. Vansant, P. Van der Voort and K. C. Vrancken, in Characterization and Chemical Modification of the Silica Surface, Vol. 93 of Studies in Surface Science and Catalysis, Elsevier, New York, N.Y. (1995), and Brinker et al., 1990, cite procedures for hydroxylation of silica surfaces by fluorination or by treatment with silane solutions. Aerogel/Xerogel-type films have been dehydroxylated by both (a) fluorination treatment, and (b) a two-step dehydroxylation method of (1) initial silane solution treatment (e.g. trimethylchlorosilane or hexamethyidisilazane (HMDS) in a solvent), and then (2) following this solution treatment with a treatment in hydrogen-containing gases (e.g. 10% hydrogen in nitrogen) at moderately high temperatures of 300-450.degree. C. The silane/forming gas(H.sub.2 in N.sub.2) treatment is discussed in U.S. Pat. No. 5,504,042 and some of the other related patents by Smith and colleagues that are referenced therein.
In the surfactant-templated films, the pores form ordered (e.g. hexagonal) arrays, with the characteristic pore size being defined by the surfactant micelle size. The surfactant templated route allows control of the porosity, pore size and pore shape using the properties of the surfactants and their interactions with the silica species. For a given level of porosity, this control in pore size and architecture and structure of the pore walls can also result in good mechanical properties. More specifically, smaller and uniform pores can impart better mechanical properties than larger and non-uniform pores. Although easier to produce (no need for controlled atmosphere to form the porosity), mesoporous surfactant templated silica films have not been demonstrated with low dielectric constant.
U.S. Pat. application 08/921,754 filed Aug. 26, 1997 by Bruinsma et al, now U.S. Pat. No. 5,922,299, describes the preparation of mesoporous surfactant templated silica films with ordered porosity of spin coating. The surfactant used was a cationic ammonium-based surfactant. A goal of this work was low-dielectric constant interlayers in microelectronic devices.
U.S. Pat. No. 5,858,457 by Brinker et al also reports a dip coating procedure for making a surfactant-templated mesoporous silica film with ordered porosity, where the surfactant used was also a ammonium-based surfactant. Brinker et al measured the dielectric constant using a mercury dot electrode on the film, reporting a value for the dielectric constant of 2.37.
However, surfactant templated mesoporous silica films prepared with ammonium surfactants and tested after pyrolysis (thermal removal) of the surfactant have been found to adsorb moisture under ambient humid conditions, and therefore do not have a low dielectric constant under the ambient humid conditions of normal manufacturing and operating conditions for semiconductor devices. No dehydroxylation steps are reported in either Bruinsma et al. or Brinker et. al.
The paper Continuous Mesoporous Silica Films With Highly Ordered Large Pore Structures, D. Zhao, P. Yang, N. Melosh, J. Feng, BF Chmelka, and GD Stucky, Advanced Materials, vol. 10 No. 16, 1998, pp 1380-1385, discusses the formation of directional or ordered large pore structures in films by dip coating silica based solutions containing non-ionic poly(alkalene oxide) triblock copolymers and low molecular weight alkyl(ethylene oxide) surfactants. Low dielectric constants (1.45-2.1) were reported for these films as measured after calcination of the films. However, a disadvantage of ordered porosity, for example hexagonal porosity, is the uncertainty in uniformity of dielectric constant in different directions on large wafers. Furthermore, no dehydroxylation procedures, that are useful for maintaining low values of dielectric constant, are reported in the paper by Zhao et al.
Thus, there is a need for a surfactant templated mesoporous silica films and method of making them that provides a dielectric constant less than 3, and that meets engineering requirements including but not limited to control of film thickness and thickness uniformity, minimum surface texture, and mechanical integrity. The dielectric constant must be relatively stable under normal operating conditions which include humid air at room temperature, and must be uniform across large wafers.